Process of making steel from pig iron



PROCESS OF MAKlll ltG STEEL FRUM PIG KRON James J. Bowden, Cortland,Qhio No Drawing. Application June 24, 1955 Serial No. 517,939

4 Claims. (Cl. 7---52) This invention relates broadly to the manufactureof low phosphorus, low nitrogen steel wherein the major component byweight of the charge is molten pig iron, and wherein oxygen gas of highpurity is blown on to the surface of the molten metal. This inventiondoes not relate to steel manufacture by either the open hearth, Bessemeror the electric furnace methods as they are commonly understood in theindustry, nor to the use of air.

The Bessemer process comprises substantially the steps of introducingmolten pig iron into a converter and supplying atmospheric air oroxygenated air through tuyeres in the bottom of the converter. Duringits passage through the metal bath the oxygen of the gas will firstoxidize those components of the pig iron which have a greater affinityto oxygen than iron, such as silicon, manganese, carbon and in the basicprocess (Thomas process) also phosphorus.

There is another and more recent process in the manufacture of steelwhich is characterized by injecting the oxidizing gas at the surface ofthe molten metal. One of the drawbacks of this latter method is due tothe fact that the slag covering the molten metal, which slag resultsfrom the action of the oxygen on the molten metal and the added flux(lime, mill scale and fluorspar), is of such a consistency that the highpurity oxygen only with difficulty is able to contact the molten metalthat is to be refined. And this condition necessitates the use of excessoxygen pressure, greater consumption of oxygen and additional time forthe reaction to go to completion, because of the difficulty resultingfrom the highly viscous slag. This has been recognized, being reportedin the Hauttman Patent No. 2,644,746, patented July 7, 1953. in fact,this patentee asserted that even when the oxygen jet was directedobliquely, it proved impossible to free the surface of the steel, sincewith an obliquely directed jet the motion of the slag is such that it iscontinuously drawn to and over the point of contact. And further in thissame patent it is pointed out that when attempting to overcome thisdifficulty of penetration of oxygen into the molten metal by increasingthe pressure, this obviously increases the cost of using oxygen to a notinconsiderable extent. Hauttman proposed to solve this problem ofsecuring direct contact between the oxygen and the molten metal byrotating the stream of oxygen about its axis to cause rotation of theslag at the point of impact.

A primary object of the present invention is to increase the extent ofcontact between the oxygen gas and molten metal, more particularly pigiron, in the manufacture of steel, by increasing the fluidity of theoverlying slag. This results in a decreased resistance of the slag tothe penetration by the oxygen gas to the molten metal surface.

Another object of the invention is to make possible the utilization ofthe oxidizing gas at a lower pressure than would otherwise be the casein securing contact of the molten metal with the oxygen gas in themanufacture of steel.

Still another object of the invention is to provide a novel steel-makingprocess characterized by a more rapid development of a basic, highlyoxidized slag at a relatively early stage of the refining period thanhas been hitherto known. This will be explained in more detailsubsequently herein.

Kiihiligld Patented Mar. ll, lhlid A more specific object of the presentinvention is to decarbonize molten pig iron in the manufacture of steelusing high purity oxygen in an improvement upon the Linz and Donawitzprocess, which process is described in an article appearing in themagazine Journal of Metals, issue of April 1955. In the processdescribed there, the flux normally consists of lime, limestone,fluorspar, dolomite, and mill scale. in this Linz and Donawitz processhigh purity oxygen is blown on top of the molten pig iron. The productsof oxidation in any manufacture of steel are silica (SiO iron oxides,oxides of manganese, magnesium, and phosphorus. The flux mentioned inthis paragraph is used in order to remove these products of oxidation.

In the M. Tenenbaum Patent No. 2,668,759, issued February 9, 1954, onsteelmaking, which is a variation of the Linz and Donawitz process, alime-containing, basic slag-forming reactant and an iron oxide-bearingmaterial, such as iron ore, sinter or mill scale is used. Other basicoxides or substances capable of yielding basic oxides, may also be used,e. g., magnesia, manganese oxide.

in the manufacture of steel by the Linz and Donawitz process theimportant thing is to have the slag fluid and reactive early in theprocess when the metal charge has as much of the initial chemical andphysical qualities as it is possible to have; this kind of conditionwill give the slag and metal ample time to do their work. in that mannerthe slag should be so conditioned that it will be possible to have theslag act as a cleanser, and remove the undesirable elements andcompounds from the molten metal underlying the slag. This means thenthat when a slag is fluid for as long as possible during thesteel-making process that it has more time to do its work; or to put itotherwise, the sooner the slag becomes fluid the easier it will be to dothe desired refining, and the more efficiently the oxygen will bebrought into contact with the molten metal for the oxidation of theelements that will eventually be found in the slag. in short, a goodslag is a fluid slag, and it is difficult to make a basic slag fluid inthe ordinary approach, using only lime, mill scale and fluorspar (whichtendency will be to make the slag thin in order to get fluidity andthinness is detrimental quality in a slag and sometimes gets out ofcontrol due to this excessive thinning).

in accordance with the present invention in making steel from molten pigiron, with or without steel scrap, using a basic lined converter thereis employed as a basic, slagforming material a fiux of lime, iron oxideand alumina, which components are present Within certain percentageranges and are either simply premixed, premixed and briquetted, orpremixed and fused, or premixed as in a slurry, such as in makingconcrete. The preferred type flux is premixed and briquetted underpressure, with the next preference premixed as a slurry; the premixedand fused would be better than the merely premixed materials. The choiceis one mainly of economy. Instead of lime, calcium hydrate, limestone ora mixture of limestone with lime can be used. The source of iron oxideis preferably mill scale, although the source could be iron ore orsinter.

The source of alumina, which i prefer to use, is a cement, preferably ahigh alumina cement, known in the trade as Lumnite. 1 Nevertheless,other source of aiu mina may be used advantageously, such as alow-alumina Portland cement, corundum, feldspar, gibbsite, cryolite,diaspore, bauxite, aluminum dress and skimmings, and red mud (a wasteproduct of the aluminum industry). Alumina, as contained in combinedform (lcaolinite), is not satisfactory, because it increases the silicacontent in the slag formed, making it necessary to add additional Atrademark of the Universal Atlas Cement Company for composition of about41% Al2Oa:38% C110 and 12% FeeOa with minor amounts of silica, etc.

lime to combine with the silica thus added, resulting in a more viscousslag because the dicalcium silicate formed has a high melting point.

The oxygen which is employed should be not less than 95% preferablypure.

The temperature of the molten iron in the charge should preferably notbe less than 2500 F.

The metallic charge which is employed, and which also is representativeof that in the Linz and Danowitz process referred to above, consists of2035% unmelted steel scrap, with the balance, 6580%, of molten pig iron,each by weight. However, as much as 100% of molten pig iron could becharged, or 60aimost 100% of molten pig iron, with the remainingpercentage being unmelted steel scrap.

In the refining of molten pig iron by contact with free oxygen, thecarbon, which usually amounts to about 4%, is eliminated as a gas; thesilicon is oxidized to silica ($0 that part of the iron which isoxidized initially appears in the slag as ferrous oxide and ferricoxide; the manganese which initially appears in the slag occurs asmanganese oxide. The iron oxide and manganese oxide which are presentfrom oxidation, as explained, combine in the presence of silica to forma silicate of iron and manganese believed to be (PeQMnOSiO Calcium oxidewhich has been added, in excess, combines with silica to form dicalciumsilicate. And at this step the iron and manganese oxides in the silicateof iron and manganese are replaced by calcium oxide which formsdicalcium silicate. Alumina combines with excess lime, and with ironoxide, to form tetracalcium' alumino ferrite. Alumina also combines withlime to form calcium aluminate. Both these types of aluminum oxidecompounds tend to lower the M. P. of the entire slag system.

The type of mineral-containing slag in either the Tenenbaum process orthe Linz and Donawitz process referred to above is difiicult to makefluid under normal operating conditions due to its high M. P. andviscosity. In order to overcome this shortcoming of high viscosity insuch a slag, I propose to add an excess of lime and a certain percentageof alumina. Because of the affinity of lime for alumina and also theaffinity of calcium ferrites for alumina, and also because of the law ofeutectics, the dicalcium silicate is fluxed. The M. P. of the entireslag system is lowered. This results in the thinning of the slag at theconventional operating temperatures, makingit fluid and basic; and itlowers the resistance of penetration of this slag to such an extent thatthe gaseous oxygen more easily penetrates this resulting slag to reachthe surface of the molten metal. The removal of the impurities dependsupon preferential oxidation.

As has been pointed out in the oxygen steel-making process developed atLinz and Donawitz (Journal of Metals, issue of April 1955), the fluxconsists of lime, limestone, mill scale, fluorspar and dolomite. Thefluidity of the slag, however, leaves much to be desired. By eliminatingthe fluorspar and dolomite and substituting for the fluorspar the properamount of alumina in a premixed flux, the desired fluidity of the slagcan be achieved. And in so doing there results conditions favorable to amore intimate contact between the high purity oxygen and the moltenmetal to be refined. As a consequence the process of steel making isspeeded up; there is a more economical use of the charged materials withincreased yield of saleable steel; the quality of the finished steel isimproved because of the close control of the process.

It has been mentioned previously that the basic flux which is added inmaking steel from pig iron according to the present invention ischaracterized by pro-mixed lime, iron oxide and alumina, each of whichis present within certain percentage ranges. The percentage compositionof the components thereof in the total flux mixture calculated I00%should be within limits herewith set out, the percentages being basedupon representations in terms of calcium oxide (lime), ferric oxide andalumina:

Percent by weight inclusive CaO 50-90 F6203 A1 0 05-15 The total weightof this fiux mixture wili vary from approximately 2% to 10%, inclusive,of the weight of the metallic charge (steel) or pig iron.

if this flux composition is simply pro-mixed and given no other physicaltreatment, this may be done by mixing the components, such as lime, millscale and a high alumina cement, in a cement mixer, or a ball or rodmill or a muiler, etc., prior to its addition to the molten pig iron.

If the flux composition is pro-mixed and briquetted with the addition ofcement, a convenient way is to form a slurry or paste from thecomponents within the percentage ranges supra using a high aluminacement as a binder, then cast, crush after hardening, and screen. It isthen added to the molten pig iron in the furnace. However, instead ofusing cement, the components can be mixed in the proportions givenabove, and then on the basis of percentage by weight:

Percent CaO 83 Fe O A1 0 3.5

Total 100.5

The amount of the flux composition is based upon the weight andcomposition of the metal charge in the furnace, particularly on thesilicon content of the iron in the charge. The approximate ingot yieldofsteel is of the charge metal weight. In a representative run by theprocess of the present invention, for 93,000 pounds of metallic Weightcharged in the basic lined furnace, consisting of 27,000 pounds of scrapiron and 66,000 of pig iron, the components by weight in the pre-mixedflux composition were 5,400 pounds of CaO, 900 pounds of Fe O and 210pounds of A1 0 These are calculated as equivalent in terms of calciumoxide, ferrosoferric oxide, i. 6., iron oxide (ous, ic) and alumina,rather than existence in this form. To reiterate, the CaO could bepresent in Whole or in part as calcium carbonate. The total based onpounds is 6,510 pounds of the flux calculated in the terms mentioned:93,000 pounds of the metallic charge. Based upon the charge metalweight, this typical flux composition would compute percentagewrse:

Percent or The flux Weight in metal terms Component lbs. charge ofperweight, cent approx.

*1 FezO3=0.69 F6203.

The basic slag-forming material characterized by aluminum oxide, calciumoxide and ferric oxide in certain percentages by weight of its totalweight facilitates the fusion of the slag upon the molten pig iron,wherein the molten iron is contacted with free oxygen at the metalsurface. The presence of alumina in the basic slagforming material isbelieved to be new. The addition of the alumina-containing, basicslag-forming material of my invention may be charged simultaneously withor immediately following the charging of the liquid metal, or

to introduce the pre-mixed alumina-containing slag-forming material tothe refining zone and then pour the molten iron over this additive inorder to facilitate mixing and distribution of the reactants whichhastens the refining of reactions.

The pro-mixed, basic slag-forming material of the present inventionfacilitates the formation of a very desirable melting point mineral. Andbeing of lower melting point than that of dicalcium silicate, whichmelts at 2l30 C., and lime, which melts at an even higher temperature,namely, 2572 O, my pre-mixed, basic slagforming additive greatly assiststhe fiuxing or solubilizing of these higher melting point materials inthe slag. Because of the alumina being present in the basic, slagformingadditive and by virtue of the law of eutectics, these two compoundsderived from alumina, namely, tricalcium aluminate and tetracalciumalumino ferrite, as Well as others of its compounds definitely assist inlowering the fluxing temperature of the lime, slag or mixture to a pointthat it is believed to be lower than that of either of these twoalumina-bearing compounds. It is also true that any otheralumina-bearing compound except its silicates will assist in the samemanner by combining with the lime and lowering its fluxing point. Thepractical aspect is that the alumina assists in putting the lime anddicalcium silicate present in the slag in solution much below thetemperature at which either of the two latter components dissolve. Bylowering the melting point of the flux, time will be saved in reachingthe temperature necessary to make the flux active in a role to removeimpurities from the molten pig iron, i. e., in reaching the M. P. of thepredesigned flux.

The premixed basic slag-forming compound will be calculated to result ina composition which, when added to the molten metal, will promptlybecome fluid. This means that the pro-mixed fiux will have a verydesirable melting point, and as further additions of it are made, thepart that is already melted will act as a receiver for the newly addedpre-mixed mineral with a consequent increased percentage rate of lowmelting point mineral being added to the system. Hence, the greater theamount of melted mineral there is in the system the quicker will theadditions which follow be liquefied and made useful in the properassimilation of the undesirable products of oxidation resulting from theaction of the oxygen on the contained silicon.

For a better understanding of this invention a specific embodiment ofthe process is herewith presented, setting out the mode of operation andthe principle of the invention.

EXAMPLE A metallic charge of 93,000 lbs., which is made'up of 27,000lbs. of steel scrap and 66,000 lbs. of molten pig iron, whosetemperature is not less than 2,500 P. is introduced into the refiningvessel in that order of addition. The analysis of the pig iron is:

4.0 to 4.5% carbon .80 to 1.20% silicon .70 to 1.80% manganese .200 to500% phosphorus Iron by difference The metal charge above is followeddirectly by the addition of a pre-mixed slag-forming material made up asfollows:

4,600 lbs. of burned lime (CaO) 2,000 lbs. of limestone 900 lbs. of millscale 210 lbs. of aluminum oxide (as dress, cement, etc.)

Immediately afterward, the high purity (99%) oxygen under pressure isintroduced through a lance substantially at the surface of the moltenmixture. This blowing of high purity oxygen at this point of time, ifsuch is to be successful, must have conditions in the vessel that makeit possible for the oxygen to easily and readily reach the surface ofthe molten metal, where the oxidation takes place. The basic, slagforming material which has been added is intermixed with the moltenmetal phase. There is an exothermic reaction when oxygen is blown on themeta; surface which results from the rapid oxidation of carbon, silicon,iron, manganese and other elements in the charge. The injection ofoxygen at the metal surface is continued until the carbon is reduced tothe desired final extent by oxidation. The resistance to penetration ofthe slag has been lowered to such an extent by the addition of my flux,which is high in alumina and also contains an excess of lime, that theoxygen gas has been able to readily reach the surface of the moltenmetal.

It should be pointed out that it is not necessary to use limestone assuch for a flux; it is only necessary to get the CaO that is needed inthe slag, regardless of where it comes from. In fact, a principal sourceof lime that I intend to use is the residue that comes from themanufacture of acetylene and the residue that comes from the manufactureof caustic soda, chlorine and soda ash or from other suitable wasteproducts that contain the right type of lime availability.

Composition of a typical pre-mixed flux charge for the manufacture of asingle heat wherein the metallic charge is 93,000 lbs. (scrap and moltenmetal) and the yield is about as ingots With relation to Flux makingconstituent the premixed flux With relation itself to the metalliccharge only, weight 93,000 Constituent Wizgght, Percent lbs., percent s.

4, 600 59. 7 4. 9 2, 000 25. 9 2. 2 Fe 0 (mill scale)-.." 900 1 11.5 .97A1203 (aluminum dross cement,

etc.) 210 2. 7 23 Total 7,110 09. s l 8.30

Same as above but calculating limestone as CaO or using any otheraddition containing CaO as CaO with relation to a slag mix and ametallic charge of 93,000 lbs.

Pounds Percent Percent CaO (equivalent) 5,600 83.6 6.0 F9304 (millscale) 900 1 13.4 97 A; (dross, cement, etc.) 210 3.1 23

Total 6, 710 100. 0 7. 20

1 =9.2% Fezoa.

ing material, and a lime-containing material, in which CaO represents50-90%, Fe O 535%, and A1 0.5- 15%, each by weight in a predeterminedamount of 2-10% inclusive of the Weight of the metal charge the majorcomponent of which is molten pig iron, injecting an oxidizing gas havinga free oxygen content of at least 95% by volume substantially at themetal surface, and intermixing the slag and metal phases and continuingthe injection of said gas substantially at the metal surface until thecarbon is reduced to the desired final extent by oxidation thereof.

2. A process of making low phosphorus, low nitrogen steel from pigirons, which process comprises introducing into a basic lined refiningzone the pig iron in molten state, a pre-mixed briquetted and sizedbasic, slag-forming composition which is an iron-oxide bearing material,an aluminum oxide bearing material, and a lime-containing material, inwhich CaO represents 50-90%, Fe O -35%, and A1 0 05-15%, each by weightin a predetermined amount of 2-10% inclusive of the Weight of the metalcharge the major component of which is molten pig iron, injecting anoxidizing gas having a free oxygen content of at least 95% by volumesubstantially at the metal surface, and intermixing the slag and metalphases and continuing the injection of said gas substantially at themetal surface until the carbon is reduced to the desired final extent byoxidation thereof.

3. A process of making low phosphorus, low nitrogen steel from pigirons, which process comprises introducing into a basic lined refiningzone a metallic iron charge consisting of 20-35% by weight unmeltedsteel scrap and from 65-80% by weight molten pig iron in molten state, apre-mixed basic, slag-forming composition which is an iron-oxide bearingmaterial, an aluminum oxide bearing material, and a lime-containingmaterial, in which CaO represents -90%, Fe O 5-35%, and A1 0 05-15%,each by weight in a predetermined amount of 2-10% inclusive of theweight of the metal charge, injecting an oxidizing gas having a freeoxygen content of at least 95% by volume substantially at the metalsurface, and intermixing the slag and metal phases and continuing theinjection of said gas substantially at the metal surface until thecarbon is reduced to the desired final extent by oxidation thereof.

4. A process of making low phosphorus, low nitrogen steel from pigirons, which process comprises introducing into a basic lined refiningzone steel scrap and molten pig iron in the ratio approximately 100%molten pig iron, the remaining percentage of the metal charge beingunmelted steel scrap, a pre-mixed basic, slag-forming composition Whichis an iron-oxide bearing material, an aluminum oxide bearing material,and a lime-containing material, in which CaO represents 50-90%, Fe O535%, and A1 0 05-15%, each by Weight in a predetermined amount of 2-10%inclusive of the Weight of the metal charge, injecting an oxidizing gashaving a free oxygen content of at least by volume substantially at themetal surface, and intermixing the slag and metal phases and continuingthe injection of said gas substantially at the metal surface until thecarbon is reduced to the desired final extent by oxidation thereof.

References Cited in the file of this patent UNITED STATES PATENTS1,890,485 Amsler Dec. 13, 1932 2,416,179 Kemmer Feb. 18, 1947 2,668,759Tenenbaum Feb. 9, 1954

1. A PROCESS OF MAKING LOW PHOSPHOROUR, LOW NITROGEN STEEL FROM PIGIRONS, WHICH PROCESS COMPRISES INTRODUCING INTO A BASIC LINED REFININGZONE THE PIG IRON IN MOLTEN STATE, A PRE-MIXED, BASIC, SLAG-FORMINGCOMPOSITION WHICH IS AN IRON-OXIDE BEARING MATERIAL, AN ALUMINUM OXIDEBEARING MATERIAL, AND A LIME-CONTAINING MATERIAL, IN WHICH CAOREPRESENTS 50-90%, FE2O3 5-35%, AND A12O3 0.515%, EACH BY WEIGHT IN APREDETERMINED AMOUNT OF 2-10% INCLUSIVE OF THE WEIGHT OF THE METALCHARGE THE MAJOR COMPONENT OF WHICH IS MOLTEN PIG IRON, INJECTING ANOXIDIZING GAS HAVING A FREE OXYGEN CONTENT OF AT LEAST 95% BY VOLUMESUBSTANTIALLY AT THE METAL SURFACE, AND INTERMIXING THE SLAG AND METALPHESES AND CONTINUING THE INJECTION OF SAID GAS SUBSTANTIALLY AT THEMETAL SURFACE UNTIL THE CARBON IS REDUCED TO THE DESIRED FINAL EXTENT BYOXIDATION THEREOF.